1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic storage apparatus.
2. Description of the Related Art
Recently, demands to increase storage capacities of HDDs (Hard Disk Drives) are increasing. As one means of satisfying such demands, a heat-assisted recording method has been proposed. The heat-assisted recording method performs recording with respect to a magnetic recording medium using a magnetic head mounted with a laser light source, by heating the magnetic recording medium by the magnetic head.
The heat-assisted recording method can reduce the coercivity of the magnetic recording medium by heating the magnetic recording medium and enable use of a magnetic material having a high crystal magnetic anisotropy constant Ku (hereinafter also referred to as a “high-Ku material”) for a magnetic layer of the magnetic recording medium. For this reason, the magnetic grain size of the magnetic layer can be reduced while maintaining thermal stability, and a surface recording density on the order of 1 Tbits/inch2 can be achieved. The high-Ku material includes ordered alloys, such as L10 type FePt alloys, L10 type CoPt alloys, L11 type CoPt alloys, or the like.
In addition, in order to isolate (or separate) crystal grains of the ordered alloy, the magnetic layer is added with a grain boundary material, such as an oxide including SiO2, TiO2, or the like, or C, BN, or the like. By employing a granular structure in which the magnetic crystal grains are separated at the grain boundary, an exchange coupling between the magnetic grains is reduced compared to a case in which the grain boundary material is not added, and a high medium SNR (Signal-to-Noise Ratio) can be achieved.
For example, En Yang et al., “L10 FePt-oxide columnar perpendicular media with high coercivity and small grain size”, Journal of Applied Physics 104, 023904 (2008) propose adding 38% SiO2 to FePt, in order to reduce the magnetic grain diameter to 5 nm. Japanese Laid-Open Patent Publication No. 11-353648 also describes that the magnetic grain diameter can further be reduced to 2.9 nm by increasing the added content of SiO2 to 50%.
In order to obtain a magnetic recording medium having a high perpendicular magnetic anisotropy and employing the heat-assisted recording method, the L10 type ordered alloy within the magnetic layer preferably has a good (001) orientation. Because the orientation of the magnetic layer can be controlled by an underlayer, a suitable underlayer is required to make the L10 type ordered alloy within the magnetic layer to have the good (001) orientation.
For example, Japanese Laid-Open Patent Publication No. 11-353648 describes that the good (001) orientation of the L10 type ordered alloy within the magnetic layer can be achieved using a MgO underlayer.
In addition, Japanese Laid-Open Patent Publication No. 2009-158054, for example, describes that the (001) orientation of the L10 type FePt magnetic layer can further be improved by forming, on a crystal grain control layer made of a Cr—Ti—B alloy or the like having a bcc (body centered cubic) structure, a MgO intermediate layer that achieves both crystal orientation control and low heat conduction.
For example, Japanese Laid-Open Patent Publication No. 2012-048792 describes an example in which Mo-5 at % Mo/Co is used for the underlayer.
In a magnetic storage apparatus that uses the magnetic recording medium employing the heat-assisted recording method, there are demands to simultaneously reduce the magnetic crystal grain size of the magnetic recording medium and sufficiently reduce the exchange coupling between the magnetic crystal grains, in order to further improve the medium SNR. As described above, adding the grain boundary material described above, such as SiO2, C, or the like, to the magnetic layer, is effective in satisfying these demands.
On the other hand, a microwave-assisted recording method has been proposed as the next-generation recording method. The microwave-assisted recording method performs recording on the magnetic recording medium by applying a high-frequency magnetic field from the magnetic head to the magnetic layer so as to tilt a magnetization direction from an axis of easy magnetization, in order to locally switch the magnetization of the magnetic layer and record magnetic information.
Similarly as in the case of the heat-assisted recording method, the microwave-assisted recording method can use a high-Ku material, such as the L10 type alloy, for the magnetic layer of the magnetic recording medium. The magnetic grain diameter of the magnetic layer needs to be further reduced in order to further improve the recording density. For this reason, the microwave-assisted recording method also requires the L10 type alloy, so that the magnetic grain size of the magnetic layer can be reduced while maintaining thermal stability.
However, when a large amount of the grain boundary material is added in order to obtain a sufficiently high medium SNR in the magnetic storage apparatus, ordering of the crystal grains (hereinafter also referred to as “magnetic layer crystal grains”) of the L10 type alloy included in the magnetic layer, such as the crystal trains of the FePt alloy, for example, deteriorates, to thereby deteriorate the crystal magnetic anisotropy constant Ku.